Liquid metal treatment prior to solidification processing is necessary for a variety of casting processes including, but not limited to, sand casting, permanent mould casting, high pressure die casting, direct chill casting, twin roll casting and the like for the purposes of grain refinement, melt cleanliness, homogeneous microstructure and homogeneity of chemical composition, dispersing and distributing of both endogenous and exogenous particles.
The existing methods for liquid metal treatment mainly include, mechanical stirring by an impeller, electromagnetic stirring, and some other methods like gas induced liquid flow.
Mechanical stirring by an impeller is a very simple way to treat liquid metals. It only provides moderate melt shearing around the impeller, but causes serious vortex in the liquid metal and serious turbulence near the liquid surface, resulting in severe entrapment of gas and other contaminants from the melt surface. There have been a number of approaches to address such problems.
U.S. Pat. No. 3,785,632 issued to Kraemer et al. discloses a process and an apparatus for accelerating metallurgical reactions. The process includes mechanical stirring at the boundary between the molten bath and the reactant, using a twin-impeller. A centrifugal force component is created when the apparatus starts stirring and causes different curvature towards the margin of the ladle which leads to the acceleration of chemical reaction between the molten metallic material and the reactants.
U.S. Pat. No. 4,743,428 issued to McRae et al. discloses a method of mechanical stirring of liquid metals for producing alloys. The process introduces an agitating device mainly to accelerate the dissolution of alloying elements and slow down the formation of dross.
U.S. Pat. No. 3,902,544 issued to Flemings et al. discloses a continuous process of treating liquid metals by mechanical stirring to obtain semi-solid metallic materials with non-dendritic primary solid. In this process three augers are introduced and located in three separated agitation zones. The augers are more effective compared to the twin blade impeller. The distance between the inner surface of the agitation zone and the outer surface of the auger is kept sufficiently small so that high shear forces can be applied to the materials in the agitation zones.
U.S. Pat. No. 4,373,950 issued to Shingu et al. introduced mechanical stirring by an impeller into direct chill casting process to purify aluminium. Aluminium melt is purified by using a mechanical stirring apparatus to break down dendrites at the interface between the liquid and the solid, and dispersing the impurity released from dendrites into the whole liquid.
U.S. Pat. No. 4,908,060 issued to Duenkelmann discloses a rotary device comprising a hollow shaft and a hollow rotor attached to the shaft for dispersing gas in molten metal. The device introduces inert gas from the top of the shaft and delivers a large volume of inert gas into the melt for degassing of liquid metals.
The inventions discussed above all involve mechanical stirring. They neither provide the high shear rate required for melt conditioning, nor avoid the problems of entrapment of gas and other contaminants from the melt surface.
U.S. Pat. No. 4,960,163 introduces a mechanical stirrer in direct chill casting for achieving fine grain structure and a partition to divide the space in the DC caster into a supply reservoir and a solidification reservoir for avoiding turbulence near the liquid surface in the supply reservoir without weakening the stirring in the solidification reservoir. A certain degree of grain refinement by this invention was achieved but the results were not consistent from batch to batch.
U.S. Pat. No. 6,618,426 issued to Ernst discloses a process of electromagnetic stirring to treat liquid metals. This process used multiple coils with different directions to reduce the turbulence near the liquid surface. However, the shearing rate by electromagnetic stirring is low and the cost of the apparatus is high.
WO 2010/032550 (Nippon Light Metal Co. Ltd) discloses a metal melt refiner for use in a ladling chamber. It is essentially a multi-blade stirrer for degassing and deslagging liquid metals. However it has very little dispersing and distributing power and the whole assembly is not suitable for direct incorporation in existing casting processes.
WO 2010/150656 (Eddy Plus Co. Ltd) discloses a distributive mixing device based on centrifugal force. It has a low shear rate and insufficient power for dispersion.
EP 1 779 924 (Prosign) discloses a disk-blade mixer for distributive mixing. It has insufficient power for dispersion.
U.S. Pat. No. 4,684,614 (Ceskoslovenska akademie ved) discloses a bladeless mixer for mixing, pumping and dissipating liquid, particularly in the food industry. It would only be suitable for low temperature applications, and could not be used to shear liquid metals.
U.S. Pat. No. 4,046,559 (Kennecott Copper Corporation) discloses a disk-blade based distributive mixer for mixing two liquids of different densities. It has insufficient power for dispersion.
US 2010/0300304 (Shimizu) discloses a hand tool for mixing small amounts of household food in the kitchen. It would not be suitable for shearing liquid metals. A further food mixer of this type is disclosed in WO 2007/042635 (Seb S.A.).
Current mechanical or electromagnetic stirring for treating liquid metals causes turbulence near the liquid surface which is harmful for most casting processes. Therefore, the stirring speed must be limited in order to achieve a relatively stable liquid surface, and consequently both effectiveness and efficiency of liquid metal treatment are compromised.
It would be advantageous, therefore, to provide a method and apparatus that can be readily applicable to existing casting processes and can provide intensive melt shearing while avoiding entrapment of gas and other contaminants from the melt surface.
The principal object of the present invention is to provide an apparatus and method for providing treated/conditioned liquid metal as feedstock for further solidification processing of metallic materials, particulate reinforced metal matrix composites (MMCs) and immiscible alloys.
Another object of the present invention is to provide an apparatus and method that can homogenise chemical compositions, disperse and distribute gas, liquid and solid phases in liquid metals or metal matrix composites (MMCs).
Still another object of the present invention is to enhance the kinetic conditions for chemical reactions and phase transformations involving at least one liquid phase.
Another object of the present invention is to provide an apparatus and method for producing high quality metallic materials or metal matrix composites (MMCs) with refined microstructure and reduced cast defects.
Yet another object of the present invention is to provide a means for dispersive mixing under high shear rate and distributive mixing with macroscopic flow in the entire volume of liquid metal without causing serious turbulence near the liquid surface.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following descriptions, embodiments and examples.